12-L1-L2-Pumps etc

Pumps, Compressors, Fans,
Ejectors and Expanders
Chapter 20
ChEN 4253 Design I
Terry A. Ring
• Moves Liquid, Creates Pressure
– Vapor bubbles
• Causes Cavitations
• Erodes Impeller
– Solids Erode Impeller
• Pump Types
– Centrifugal
– Positive Displacement
• Piston
• diaphragm
• Pump Power = Q*ΔP = brake (delivered) (horse) power
from motor
Centrifugal Pumps
• Two Basic Requirements for TroubleFree Operation of Centrifugal Pumps
– no cavitation of the pump occurs throughout
the broad operating range
– a certain minimum continuous flow is always
maintained during operation
• Pump around loops
Reduced Flows
• Unfavorable conditions which may occur
separately or simultaneously when the pump is
operated at reduced flows
• Cases of heavy leakages from the casing, seal, and stuffing
• Deflection and shearing of shafts
• Seizure of pump internals
• Close tolerances erosion
• Separation cavitation
• Product quality degradation
• Excessive hydraulic thrust
• Premature bearing failures
Centrifugal Pump
Electric Motor
Centrifugal Pump
Centrifugal Pump
• Converts
energy to
Converts Kinetic Energy to
Pressure Energy
Different Types of Pump Head
• Total Static Head - Total head when the pump is not running
• Total Dynamic Head (Total System Head) - Total head when the
pump is running
• Static Suction Head - Head on the suction side, with pump off, if the
head is higher than the pump impeller
• Static Suction Lift - Head on the suction side, with pump off, if the
head is lower than the pump impeller
• Static Discharge Head - Head on discharge side of pump with the
pump off
• Dynamic Suction Head/Lift - Head on suction side of pump with
pump on
• Dynamic Discharge Head - Head on discharge side of pump with
pump on
Pump Head
• The head of a pump can be expressed in metric
units as:
• head = (p2 - p1)/(ρg) + (v22- v12)/(2g) + (z2-z1)
• where
• h = total head developed (m)
• p2 = pressure at outlet (N/m2)
• p1 = pressure at inlet (N/m2)
• ρ = density of liquid (kg/m3)
• g = acceleration of gravity (9.81) m/s2
• v2 = velocity at the outlet (m/s)
Pump Efficiency
• Centrifugal Pump
Pump Performance Curves
Pump Design Scaling
• Pump Flow rate
• Q2 = Q1 x [(D2xN2)/(D1xN1)]
• Pump Head
• H2 = H1 x [(D2xN2)/(D1xN1)]2
• Pump Brake Horse Power
• BHP2 = BHP1 x [(D2xN2)/(D1xN1)]3
– D = Impeller Diameter
– N = specific speed
Net Positive Suction Head-NPSH
• Pumps can not pump vapors!
• The satisfactory operation of a pump
requires that vaporization of the liquid
being pumped does not occur at any
condition of operation.
Net Positive Suction Head
Required, NPSHR
As the liquid passes from the pump suction to the eye of the impeller, the velocity
increases and the pressure decreases. There are also pressure losses due to
shock and turbulence as the liquid strikes the impeller. The centrifugal force of the
impeller vanes further increases the velocity and decreases the pressure of the
liquid. The NPSH required is the positive head (absolute pressure) required at the
pump suction to overcome these pressure drops in the pump and maintain the
liquid above its vapor pressure.
Net Positive Suction Head
Available, NPSHA
Net Positive Suction Head Available is a function of the system in which the
pump operates. It is the excess pressure of the liquid in feet absolute over its vapor
pressure as it arrives at the pump suction, to be sure that the pump selected does
not cavitate.
Head to Feed Pump
To overcome suction head
Subcooling before Pump
Cool a few Degrees
To overcome suction head
Piston Pumps
Gear Pumps
Lobe Pumps
• food applications,
because they
handle solids
without damaging
the pump.
• Particle size
pumped can be
much larger in
these pumps than
in other PD types
Screw Pump
Positive Displacement Pumps
• Piston Pumps
• Gear Pumps
– Lobe Pumps
• Diaphragm Pumps
– The lower the speed of a PD
pump, the lower the NPSHR.
Pump Costs
• Cost based upon Size Factor
– Centrifugal Pump
• S=QH1/2
– Gear Pump
• S=Q
– Piston Pump
• S= Power (brake)
• Must cost Electric Motor also
• S=Pc=PB/ηM
– Centrifugal
– Others
• Piston
• Lobed
• Screw
– Methods of Calculation in Simulators
• Polytropic, PVk-1/k= constant,
– Polytropic - This model takes into account both a rise in temperature in the gas as well as
some loss of energy (heat) to the compressor's components. This assumes that heat may
enter or leave the system, and that input shaft work can appear as both increased
pressure (usually useful work) and increased temperature above adiabatic (usually losses
due to cycle efficiency). Compression efficiency is then the ratio of temperature rise at
theoretical 100 percent (adiabatic) vs. actual (polytropic). (k-1)/k = polytropic coefficient
• Isentropic, s(T1,P1)=s(T2,isentropic,P2)
• Theoretical Power
– Powerisentropic= FlowRate*(h2,isentropic-h1)
• Efficiency ηs =Powerisentropic/Powerbrake
• ηs = (h2,isentropic-h1)/(h2-h1)
– Cost of Compressors
• Size Factor is Compressor Power
T 2  T1 
k 1
 P2 
T1 
 P 
 1
Positive Displacement Compressor
Positive Displacement Compressor
Centrifugal Compressors
• Rotors
• Stators
• Jet
Piston Compressor
• Reverse of Compressor
• Let flow produce shaft work
• Types
– Centrifugal
– Positive Displacement
• Piston
• Lobed
• Screw
– Methods of Calculation in Simulators
• Polytropic, PVk-1/k= constant,
• Isentropic, s(T1,P1)=s(T2,isentropic,P2)
• Theoretical Power
– Powerisentropic= f*(h2,isentropic-h1)
• Efficiency ηs=Powerbrake/Powerisentropic= (h2-h1) /(h2,isentropic-h1)
– Cost
• Size factor = Power
Fans and Blowers
• Types
– Centrifugal (103-105 acfm, P=1-40 in H2O)
• Backward Curved
• Straight radial
– Vane Axial
– Tube Axial
• Cost of Fans and Blowers
– Size factor = Volumetric Flow Rate
– Motor
Choice to Increase Pressure
• Heuristic 34
– Use a Fan
• Atm to 1.47 psig
– Use a Blower
• < 30 psig
– Compressor (or staged system)
• > 30 psig
• Heuristic 34 - Number of Stages
– Up to a Compression ratio 4 for each stage
• With intercooler between stages (ΔP=2 psi)
– Equal Hp for each stage (equal compression ratio)
Producing Vacuum
Steam Ejector
Producing Vacuum
• Types
– Ejector - advantage = large volumetric flow rate
• Multi-Stage with interstage condensers
– Liquid (Oil) Ring Vacuum Pump
– Dry Vacuum Pump (rotary screw, lobe) (advantage =low
pressure) Designs similar to Expanders
• Design for
– Flow Rate at suction plus
– Air Leakage Rate
• Function of pressure and Volume of vessel
• Cost
– Size factor = Flow Rate at suction
– Motor for pumps
• Produces Vacuum
• Provides Low Pressures for
Distillation Columns
• Fluid (P ≥ Psat)
– Steam
• for suction pressure below 100 mbar
absolute, more than one ejector will be
used, with condensors between the
ejector stages
– Air
– Water
• Collects Particles in Gas Stream
– Venturi Scrubber

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